In the process of electrostatographic printing, a photoconductive member is uniformly charged and exposed to an image of an original document. Exposure of the photoconductive member provides an electrostatic latent image corresponding to the image of the original document. The latent image is developed by applying a developer material (developer) to the photoconductor over the area defined by the latent image frame. A typical two-component developer material is comprised of toner particles which adhere triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image frame to form a developed image, which is subsequently transferred and fused to a copy sheet.
In electrostatographic copiers and printing machines, magnetic rollers are employed in some process stations, such as the developing station and the cleaning station, for transporting the developer. For example, in commonly-assigned U.S. Pat. No. 4,473,029, there is disclosed a development system comprising a magnetic brush roller and a two-component development material (developer). The magnetic brush applicator comprises a cylindrical sleeve having a cylindrically-shaped multi-pole magnetic core piece. The developer comprises a mixture of thermoplastic toner particles and hard magnetic carrier particles of high coercivity (&gt;500 Oersted) and high remanence (&gt;500 Gauss). Such materials are considered hard magnetic materials as opposed to pure iron, for example, which is a soft magnetic material. During rotation of the magnetic core piece, the developer is transported along the sleeve's outer surface from a reservoir to a development zone. The developer contacts the latent electrostatic image and toner particles are stripped from the carrier particles to effect image development. Following image development, these carrier particles are stripped from the sleeve and returned to the developer reservoir for toner replenishment.
At the cleaning station, a layer of carrier granules adheres to the sleeve of another magnetic roller for movement to the photoconductive member. Residual toner particles are attracted to the carrier granules for collection and removal (cleaning).
There are generally two types of magnetic cores: multiple-bar cores or single-piece cores. As illustrated in FIG. 1, a conventional multiple-bar core 10 is typically constructed from a group of permanent magnets 12 (each of which is typically bar-shaped) that are assembled upon a central support 15. The magnets 12 provide alternating, radially-oriented magnetic north N and south S pole faces. The magnetic field is therefore modeled as a circuit of magnetic flux lines 14F which originate from within the magnet 12, exit at a north pole face N, and return to the magnet by entering a south pole face S. Flux lines H from a given magnet generally pass through the adjoining magnets that are situated within the magnetic flux circuit. Each magnet has an appropriate pole face orientation so as to reinforce the magnetic flux 14F. A common choice of material to constitute the magnets 12 is a compacted (pressed) hard magnetic ferrite or other hard magnetic material, such as samarium cobalt.
Single-piece core construction is typically of a cylindrical body formed of powdered hard magnetic material compounded in a binder. For example, an injection-moldable compound of barium or strontium ferrite powder in a nylon binder is often used. Alternating magnetic poles N and S are formed in the body during the molding process, or shortly thereafter. A typical pole configuration may be twelve alternating poles situated at the external, or circumferential, surface of the core.
Examples of the above types of magnetic cores are present in the prior art. U.S. Pat. No. 4,806,971 discloses a single-piece core formed of a moldable plastic material containing a comminuted ferrite. Longitudinal, angularly-spaced poles are produced in the material during the molding process. U.S. Pat. No. 4,558,294 discloses a method for assembling a magnetic roll in which a plurality of plastic magnetic bars are bonded to a polygonal supporting base and each other. U.S. Pat. No. 4,580,121 describes a magnetic roll having an impeller-shaped support and a plurality of rubber matrix magnetic bars mounted on the support at desired locations. U.S. Pat. No. 4,608,737 discloses a magnetic developer roll made from rectilinear ribs of plastic magnets. U.S. Pat. No. 4,638,281 discloses a magnetic roll in which permanent magnetic bars are secured to a supporting base by an injection-moldable plastic. U.S. Pat. No. 4,823,102 discloses a magnetic roll having a central portion with a plurality of spaced radial fins; a magnet is secured in each space between the fins.
The magnetization of either core type (multiple bar or single piece core) is accomplished by the application of an intense, pulsed magnetic field. A simplified model of the magnetization of a conventional magnet 12 is illustrated in FIG. 2. Magnetic lines of force H generated by a magnetizing fixture (not shown) align the internal magnetic domains D of the crystalline structure in the body of the magnet 12. Each magnetic domain D may be modeled as a disk-like region having a magnetic dipole moment which aligns with the applied magnetizing field. After the magnetizing force H is removed, a proportion of the oriented domains are then magnetized according to the remanence of the material, to thus give the magnetized piece its permanent magnetic properties.
A single-piece core is typically magnetized in an electromagnetic fixture (not shown) having a plurality of electromagnet pole pieces with current-carrying windings thereon. The magnetizing pole pieces are arranged radially around the exterior of the mold to induce radial magnetizing field lines similar to the force lines H in FIG. 2. The magnetizing field thereby extends radially from the magnetizing pole pieces through the core.
Single-piece magnetic cores are economical only when produced in quantities greater than about 10,000 cores. The initial cost of production is very high because of the cost of the injection mold, the magnetizing fixture, and the pulsed power supply that energizes the magnetizing fixture. Because there is little space between the magnetizing pole pieces (in the magnetizing fixture), it is difficult to include adequately-sized induction coils on the pole pieces. The magnetizing field intensity induced at the desired point in the single-piece core is therefore limited by the number of wire turns on each coil, and the remnant external magnetic field strength of the core may be insufficient for some uses.
For a core quantity of less than about 10,000 units, a multiple-bar core assembled from bar magnets is usually more economical to produce than the single-piece core. Accordingly, prototype magnetic rollers are often built by assembling magnets 12 of pressed ferrite or ceramic magnetic material to form the core 10. Such ceramic or ferrite bar magnets are typically composed chiefly of hard magnetic material and little or no binder. Such a composition is usually selected because it is capable of a magnetic field strength that is higher than that available from a composition of a magnetic material bound with a non-magnetic binder.
The peak magnetic field strength at a predetermined distance from the conventional magnetic core 10 is typically available over the center of a magnetic pole. External field strength is a significant criterion in selecting a magnetic core for use in a magnetic roller. The deposition of carrier from a magnetic roller onto the photoconductor (known as developer pick-up or DPU) is quite undesirable and generally increases as the field strength decreases.
Permanent magnets formed from bound magnetic compounds offer less magnetic field strength because a certain volume of the formed piece is typically composed of non-magnetic binder. Ceramic bar magnets typically have little or no binder and thus have a higher field strength. The remanence (B.sub.r) of a compound of a magnetic ferrite (e.g., barium ferrite) in a moldable binder is, for example, 2650 Gauss; the remanence of barium ferrite alone may be as high as 3800 Gauss. For convenience, the term "ceramic magnets" will be used herein to differentiate magnets of all or substantially all ceramic or ferrite magnetic material, from magnets formed of a magnetic material composition that includes one or more non-magnetic binders. Also, non-ceramic magnets are typically formed via methods such as injection molding or extrusion.
Ceramic magnets are not easily shaped for assembly into a cylindrical core. Ceramic bar magnets are typically formed from dry or wet ferrite powder (slurry) that is pressed into a form, magnetized, and then fired. The slurry undergoes considerable volumetric shrinkage during firing, and thus a fired piece typically requires costly machining to achieve specific dimensions. Hence, the slurry is often formed into slabs, fired, and then cut into rectilinear bars. Ceramic bar magnets may then lack the fan-shaped cross-section that is preferred for their assembly into a smoothly-continuous cylindrical core. Significant air gaps between the assembled bars must be tolerated, and the low permeability of each air gap lowers the level of the magnetic field intensity that can be produced by such a core. Because ceramic magnets are quite hard and brittle, their exposed edges are prone to cracking and chipping.
Ceramic magnets can be cost-effective when assembled as a prototype but they are too expensive to use in a high-quantity production run of cores. On a per-unit basis, the production cost of a machined ceramic bar is approximately ten times the production cost of an equivalently-shaped injection-molded bar magnet.
However, the use of non-ceramic (injection-molded, for example) magnets in a magnetic core has been limited because the external field strength of such a core is insufficient to control DPU in some applications. Similarly, the use of injection-moldable compounds in single-piece cores has been limited. Therefore, a magnetic core is often composed of ceramic bar magnets rather than of bar magnets composed of magnetic material in a binder.